Blender 2.8 How to make the object color in viewport look excatly as it is?
EDIT (NEW SOLUTION): 
I was browsing the bug reports and found the answer. 
Set your Workbench viewport to Flat - Vertex Color. 
In the Overlay Settings set the Vertex Paint opacity to 0, 
this way you are going to avoid the darkening effect of your vertex colors (if not they multiply by themselves)



This is an issue I am fighting right now with.
The vertex colors viewport rendering is a mess and the solution I found is to tweak workbench to do the dirty work.

For painting if you want to have 1:1 vertex colors set your Solid view to:


Flat
Single (White)


(Object works too if your object color is set to white, you can then at least have outlines for complex scenes)

To see the whole scene with vertex colors I managed by setting Workbench's Rendered view as:


Flat
Vertex (note if you go in Vertex Coloring mode everything becomes darker)


And in color management


sRGB
Standard


So to work on on a scene with vertex colors you will have to swap between Rendered view and Solid view - It's terrible for color picking. I hope someone will fix this.